1. Field of the Invention
The present invention pertains to a method of operating a near infrared spectroscopic analyzer particularly adapted for analyzing hydrocarbon fluid compositions.
2. Background
Near infrared spectroscopic analyzers are used in many petroleum refinery process applications for measuring several properties of refinery process fluids including crude oils, gasolines and other products. The basic methodology involving the use of near infrared (NIR) spectroscopic analyzers includes obtaining the near infrared spectra of a training set of sample fluids for which the properties of interest have been measured by traditional techniques. For example, in the case of using an NIR analyzer to measure octane numbers of motor gasoline, a set of gasoline samples would be measured by octane engines to obtain the requisite octane numbers. Models that relate the measured properties of the samples to their spectra may be generated using a computer and regression analysis software. These models would then allow the prediction of the properties of the unknown samples directly from their spectra measured by the spectrometer.
The model predictions are accurate as long as the unknown samples are sufficiently similar to the training set samples. Accordingly, when an "outlier" sample is detected, it can be for many reasons such as, the sample composition lies outside the training set composition range, the process stream has fundamentally changed due to an engineering design change, the near infrared analyzer instrument has undergone some mechanical or electrical change or the primary reference measurement is inaccurate, for example. With this many conditions which could result in a measurement of an outlier, it has been deemed desirable to develop a method which would assist in determining which variable in the above-mentioned set of opportunities for inaccuracy has developed, or if the measured sample of fluid is indeed a true outlier. It is to this end that the present invention has been developed with a view to providing a method for operating a near infrared spectrometer to analyze certain properties of fluids such as properties of motor gasoline and diesel fuels, for example.